The invention relates to an exhaust gas purification catalyst with improved hydrocarbon conversion, which contains metals from the platinum group as its catalytically active components and comprises two superposed functional layers applied to a support. Another aspect of the invention relates to the manufacture and production of the catalyst. Still another aspect of the invention involves the treatment of exhaust gases with the catalyst.
Internal combustion engines emit pollutants, which have to be converted into harmless compounds by suitable exhaust gas purification catalysts. The main pollutants are carbon monoxide, nitrogen oxides and unburned hydrocarbons. They may be converted into water, carbon dioxide and nitrogen by catalysts containing metals from the platinum group. The different types of catalyst are three-way catalysts, oxidation catalysts and reduction catalysts. The boundary between these catalysts is not fixed and depends in particular on the composition of the exhaust gas.
Three-way catalysts are capable of converting all three of the above-mentioned pollutants at the same time under stoichiometric exhaust gas conditions. The nitrogen oxides are reduced to nitrogen with simultaneous oxidation of carbon monoxide and hydrocarbons. As their essential catalytic component, three-way catalysts contain rhodium combined with platinum and/or palladium. A catalyst of this type containing rhodium and palladium is described in DE 38 35 184 C2, for example. The presence of rhodium in the three-way catalyst is important for its reducing function. Catalysts which contain only platinum and/or palladium have only an unsatisfactory reductive effect and are therefore used predominantly as oxidation catalysts. When they have reached their operating temperature, modern three-way and oxidation catalysts are capable of converting the pollutants concerned into harmless products at rates of more than 70%. However, a significant problem is still the cold-starting of internal combustion engines. During the so-called cold-start phase, which covers approximately the first 100 seconds after the engine is started, the exhaust gas purification catalyst is still cold and therefore inactive. As the exhaust gas temperature rises the catalyst warms up. The increasing pollutant conversion activity of the catalyst is characterized by the light-off temperature of the respective pollutant. This is the temperature at which the pollutant concerned is 50% converted. The light-off temperatures of modern catalysts range from 200 to 400.degree. C.
The main pollutant components during the cold-start phase are unburned hydrocarbons. Various testing methods have been developed to assess the purification effect of exhaust gas purification catalysts. One testing method which is frequently used is the so-called FTP 75 test, which was developed in the USA. In Europe the standard testing method is the ECE testing method.
The FTP 75 test extends over a period of 2500 seconds after an internal combustion engine is cold-started and is subdivided into three stages. The pollutants emitted during these three stages are collected in three bags and are then analyzed. The first stage covers the real cold-start phase and is concluded after 500 seconds. The following two stages simulate varying load conditions and a warm start.
For the ultimate assessment of an exhaust gas purification system, which may consist of several catalysts and adsorbers, the pollutants collected in all three bags are evaluated. It has been shown that substantial improvements in the overall assessment may be obtained in particular by improvements in pollutant conversion during the cold start phase. The conversion rates of the catalysts at their operating temperature allow at most only slight improvements, which are barely able to influence the overall assessment made according to the FTP 75 testing method. Thus, an internal combustion engine emits during the first 100 seconds after a cold start approximately two thirds of the total amount of hydrocarbons emitted during the FTP 75 test.
To reduce these hydrocarbon emissions, various combinations of hydrocarbon adsorbers and catalysts have been proposed.
U.S. Pat. No. 5,078,979 describes a method of exhaust gas purification in which the exhaust gas is firstly conveyed through a hydrocarbon adsorber and then through a catalyst. The hydrocarbons contained in the cold exhaust gas are adsorbed by the adsorber until the latter has reached a temperature of approximately 150.degree. C. Above this temperature the hydrocarbons begin to desorb again from the adsorber and are converted by the catalyst, which is warmer by then, into harmless products. Suggested adsorbers are molecular sieves (zeolites), which adsorb the hydrocarbons preferentially over water vapor which is also contained in the exhaust gas and exhibit high temperature stability.
A disadvantage of such a system is the fact that the adsorber itself takes heat from the exhaust gas during the warming-up phase, which is then unavailable to the catalyst connected downstream, such that the latter is slower to heat up than when no adsorber is connected upstream. Although combining a spatially separate adsorber and catalyst reduces the emission of hydrocarbons during the cold start phase, such an arrangement performs less well in the overall assessment according to the FTP 75 testing method than a single three-way catalyst, since the hydrocarbons adsorbed first are desorbed once the desorption temperature is exceeded and pass through the still insufficiently active catalyst to a considerable extent without being converted into harmless components.
To improve this situation, DE 42 39 875 A1 proposed the combination of an oxidation catalyst with a hydrocarbon adsorber on one support. Oxidation catalysts and hydrocarbon adsorbers are applied to the support in the form of superposed coatings, wherein the adsorber coating lies on the catalyst coating and comes into direct contact with the exhaust gas.
The oxidation catalyst contains platinum and/or palladium as catalytically active components. The adsorber contains a mixture of a dealuminized Y-zeolite and a zeolite ZSM5, wherein the Y-zeolite exhibits an Si/Al ratio greater than 40 and the zeolite ZSM5 exhibits an Si/Al ratio greater than 20.
EP 0 716 877 A1 likewise proposes the combination of a hydrocarbon adsorber with a catalyst in the form of two superposed coatings. The catalyst coating lies on the adsorber coating and contains as its catalytically active components one or more platinum group metals from the group comprising platinum, palladium, rhodium, ruthenium and iridium on aluminum oxide, cerium oxide and zirconium oxide. A zeolite with a weight ratio of silicon dioxide to aluminum oxide of more than 300 is used as the adsorber material. A monolithic honeycomb body of cordierite with parallel flow channels for the exhaust gas acts as the support. The adsorber coating is applied as a first layer directly onto the inner wall surfaces of the flow channels. Because of the poor adhesion of zeolite coatings to this substrate, the adsorber layer contains, in addition to zeolite, the same amount of colloidal silicon dioxide as a binder. Owing to this high content of colloidal material, the risk arises that the pores of the zeolite will become partially blocked and its adsorption capacity will therefore be impaired.
The solutions proposed are unsatisfactory from the point of view of their hydrocarbon suppression over all three stages of the FTP 75 test. The hydrocarbon adsorption during the first stage is adequate, but there are shortcomings in the dynamic pollutant conversion during stages 2 and 3 of the FTP 75 test. In particular, the ageing stability of the pollutant conversion is also unsatisfactory.